A MIMO system increases the performance and communication capacity of a wireless communication system. MIMO employs multiple transmission antennas and multiple reception antennas to enhance data transmission/reception efficiency, and hence, is also called a multiple antenna system. MIMO techniques may include transmit diversity, spatial multiplexing, and the like. Transmit diversity is a technique that transmits the same data from multiple transmission antennas to thus enhance a transmission reliability. Spatial multiplexing is a technique that simultaneously transmits different data from multiple transmission antennas to thus transmit high speed data without increasing a bandwidth of a system.
The capacity of a MIMO channel increases with the number of antennas. The MIMO channel may be viewed as multiple independent channels. Each independent channel may be a spatial layer. The expression “rank,” or transmission rank, refers to the number of independent data streams, or spatial layers, transmitted over a wireless channel. If a transmitter is equipped with Nt antennas and the receiver has Nr antennas, the maximum spatial multiplexing order (the number of streams) is Ns=min (Nt, Nr) if a linear receiver is used. This means that a maximum of Ns streams can potentially be transmitted in parallel, ideally leading to an Ns increase of the spectral efficiency (the number of bits per second and per Hz that can be transmitted over the wireless channel). For example, if a transmitter has two antennas, then rank one means that one (the same) stream is sent over the two antennas and rank two means that two different streams are sent over the two antennas. But if a transmitter has four transmit antennas, then rank two means that two streams for example may also be sent over all four antennas. The number of antennas does, however, limit the maximum number of streams that can be transmitted.
In the complex situation of multiple, and dynamically changing, ranks it becomes increasingly difficult to optimise power usage for the different channels. It would be highly beneficial if available transmission power is used more efficiently by the UE in a MIMO capable UE.